The present invention relates generally to the manufacture of motor vehicle brake drums and, in particular, to the manufacture of composite brake drums.
Virtually all wheeled vehicles are provided with a brake system for selectively inhibiting the rotation of the wheels and, therefore, slowing the movement of the vehicle. To accomplish this, a typical vehicle brake system includes a friction brake assembly which is provided at one or more of the vehicle wheels. Upon actuation by a driver of the vehicle through manual movement of a brake pedal and an associated pneumatic or hydraulic actuating system, the friction brake assemblies are effective to inhibit the rotation of the vehicle wheel.
Such vehicle friction brake assemblies are generally classified into two types, namely, drum brake assemblies and disc brake assemblies. In a drum brake assembly, a hollow cylindrical drum is secured to the wheel of the vehicle for rotation therewith, while a brake shoe assembly is secured to the nonrotatable components of the vehicle. The brake shoe assembly includes a pair of arced friction shoes which are operatively connected to a pneumatically or hydraulically actuated piston. The friction shoes are disposed within the hollow drum adjacent to an inner cylindrical surface (A.K.A. braking surface) thereof. The friction shoes are normally spaced apart from the braking surface of the drum. When the driver of the vehicle manually moves the brake pedal, the piston is actuated to move the friction shoes apart from one another into frictional engagement with the braking surface of the drum. As a result, rotation of the drum and its associated wheel are inhibited.
In the past, drums and rotors of the type described above have been formed from gray iron using a conventional "as-cast" method. The "as-cast" method simply involved casting molten gray iron into the desired shape of the drum or rotor and subsequently cooling, followed only by cleaning and machining when necessary. Thus, the "as-cast" method has been found to be desirable because it is a relatively simple and inexpensive method to perform. Also, gray iron has been found to be an acceptable material to use in the "as-cast" method because it provides the resultant drums and rotors with sufficient mechanical and physical properties for use in the frictionbrake assemblies, such as hardness, strength, wear resistance, thermal conductivity, and the like.
Motor vehicle brake drums are often formed entirely of cast gray iron. However, while cast gray iron is the preferred material for the braking surface of the brake drum, a full cast brake drum is relatively heavy. Another problem with fully cast iron brake drums are that they are brittle. This brittleness can lead to heat check crack-through and breakage, resulting in brake drum disintegration.
At least in response to the desire for lighter vehicle components, composite brake drums have been developed which comprise of a stamped steel drum back and a rolled steel annular band surrounding a cast iron braking surface. These drums are mostly steel. In a typical composite cast iron is only used to form the braking surface, the rest of the brake drum is composed of steel.
The structure and method of manufacture of such composite brake drums are known. In accordance with the conventional practice, these composite brake drums are manufactured by first forming an annular band from sheet steel stock which is then rolled to the desired configuration, typically having a radially inwardly-turned end flange and a plurality of outwardly directed strengthening ribs. In addition to providing strength, the strengthening ribs also increase the surface area to dissipate heat and improve the cooling capabilities of the brake drum.
With the annular steel band externally fixed, molten gray iron is centrifugally cast therein to form a metallurgically bonded composite brake ring. After the ring has cooled, a steel drum back is generally welded to the flange. The radially inwardly directed braking surface of the ring is then finish machined.
While the conventional centrifugal casting method yields quality composite brake drums, it would be advantageous to provide an improved method of manufacturing a brake drum which is less expensive then the current centrifugal casting. It is also advantageous to produce such a composite brake drum which will operate under cooler and cleaner conditions to increase the useful life of the brake drum.